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What substitution should be used to rewrite [tex]$4x^4 - 21x^2 + 20 = 0$[/tex] as a quadratic equation?

A. [tex]u = x^2[/tex]
B. [tex]u = 2x^2[/tex]
C. [tex]u = x^4[/tex]
D. [tex]u = 4x^4[/tex]

Answer :

To rewrite the equation
[tex]$$4x^4 - 21x^2 + 20 = 0$$[/tex]
as a quadratic equation, we first notice that the term [tex]$x^4$[/tex] can be expressed as [tex]$(x^2)^2$[/tex]. This observation suggests a substitution that simplifies the equation.

Step 1: Let
[tex]$$u = x^2.$$[/tex]

Step 2: Express [tex]$x^4$[/tex] in terms of [tex]$u$[/tex]. Since [tex]$x^4 = (x^2)^2$[/tex], we have
[tex]$$x^4 = u^2.$$[/tex]

Step 3: Substitute [tex]$u$[/tex] into the original equation. The equation becomes:
[tex]$$4u^2 - 21u + 20 = 0.$$[/tex]

This is now a quadratic equation in terms of [tex]$u$[/tex].

Thus, the substitution that should be used is:
[tex]$$u = x^2.$$[/tex]

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